The present invention relates to sealing apparatus particularly, but not exclusively, to sealing apparatus for sealing one or more electrodes in a cartridge of an electrical discharge machining (EDM) apparatus.
EDM is used widely to machine perforations or cavities in electrically conductive metals. The process is used, for example, in the production of bores and features, both cylindrical and otherwise shaped, in gas turbine engine components and in turbine blades and veins especially. In this application the diameter of the bores is small, typically ranging from 0.25 mm to over 1.0 mm and there is a high aspect ratio, i.e. the ratio of bore depth to diameter. A typical aspect ratio is 20.
It has been found advantageous when creating high aspect holes (i.e. aspect ratios greater than say 4) to utilise tubular electrodes and to flow dielectric fluid, normally deionised water, through the tubular electrodes towards the workpiece being machined. The dielectric fluid assists flushing eroded debris from a bore being machined whilst at the same time cooling the electrode. This arrangement enables significantly faster drilling whilst at the same time achieving the required metallurgical criteria. The dielectric fluid utilised is under sufficient pressure to allow fluid to pass through the length of the tubular electrode towards the workpiece. This requirement involves providing fluid under a pressure of typically 1000 psi, that is approximately 6900 kN/m2, at the inner end of the electrode or electrodes.
It has been found that as well as flowing through each electrode, fluid tends also to discharge around the outer surfaces of the electrodes and one objective of the invention is to reduce or eliminate this discharge.
In existing electrode cartridges a cover plate is bolted or otherwise secured to the base of the cartridge which includes a plurality of grooves each of which receives a respective electrode and which guide the electrodes to produce the desired arrangement of holes. Currently, the replenishment of the electrodes is performed manually. By replenishment is meant resetting the electrode tips in their correct positions relative to the workpiece by drawing further lengths of electrode from the electrode cartridge. A further objective of the invention is to facilitate the automatic replenishment of electrodes.
According to the present invention there is provided a sealing apparatus for effecting a temporary seal against an object comprising a chamber with an elongate object passing from an inlet aperture to an outlet aperture in the chamber, characterised by a resilient seal disposed within the chamber adjacent one of said apertures, and a piston for compressing the seal, which is located between the respective apertures and adjacent the seal, the piston being movable between a non-sealing position in which the seal is relaxed and a sealing position in which the piston engages and compresses the seal thereby creating lateral deforming movement of the seal material to bring seal material into sealing contact with the object to be sealed and means for displacing the piston between the non-sealing and sealing positions.
Preferably, the resilient seal is made of generally homogenous material.
The piston may include one or more passages for receiving respective elongate objects, so that the object or objects against which a temporary seal is to be made can extend straight through the sealing apparatus.
The piston may be stepped, the step being engageable against a shoulder of the enclosure to limit axial movement of the piston and hence the amount by which the seal material is compressed.
The means for displacing the piston from its non-sealing to its sealing position may be fluid operated, for example, by the use of a high pressure fluid, typically water, acting on the piston head. Alternatively, the piston may be displaced electrically, for example, by means of a solenoid. Alternatively again, the piston may be displaced pneumatically.
The object against which a temporary seal is to be made may be an electrode or a plurality of electrodes of an EDM apparatus. When a plurality of electrodes are employed, they may be arranged in spaced relationship.
When the sealing apparatus is applied to EDM, both the seal material and the piston are made from an electrically insulating material since, in use, an electrical potential is applied to the or each electrode. In this instance, the piston is preferably of polyurethane or acetal. Further in this application, a support may be interposed between the relevant aperture of the chamber and the seal. A bore or bores in the support is of less diameter than that of the or each aperture in the seal in its relaxed condition, i.e. when the piston is in a non-sealing position, so that, in use, the support serves to prevent seal material being extruded out of the chamber through the annular space between the object and the aperture in the chamber.
Part of the enclosure may be provided as a chamber in a block containing the electrode or electrodes and part in a pressure cap rigidly secured to the block. The depth of the part of the chamber within the block may be somewhat greater than that of the part within the cap so that a shoulder is provided at the junction between the two. This shoulder may serve as a stop to limit movement of the piston and so limit the amount by which the seal material is compressed.
An xe2x80x98Oxe2x80x99 ring seal may be provided in that part of the cylindrical wall of the chamber in the block to serve as a fluid seal when fluid is used to displace the piston. The piston may be displaced from its non-sealing position by the application of high pressure fluid, for example, water under a pressure of 1000 psi, that is approximately 6900 kN/m2.
Also according to the present invention there is provided a method of effecting a temporary seal against an object which comprises placing the object adjacent a resilient seal, moving a piston located for sliding movement against the seal, the piston being moveable between a non-sealing position in which the piston lies displaced from the seal and a sealing position in which the piston engages and compresses the seal thereby creating deforming movement of seal material to bring seal material into sealing contact with the object.